@Rapida your "savee" could still die after you killing the last foe, due to poison, which has DoT effect.
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DrFishJun 1 '12 at 7:02

1

@Bora that's true, but I've seen it in situations where there were no DoTs being used. I think Phillip is on track by saying it is based on per monster rather than last monster in a combat.
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RapidaJun 1 '12 at 8:07

This answer is up to date as of 05/18/2014, game version 2.0.5, and comes from having farmed the achievement right now.

Getting a Savior callout requires:

having an ally below 5% life,

killing the last monster that was aggressive towards your wounded ally.

The "victim" player should have two sets of items: one with maximum Life, another with no Vitality at all. Swapping to and from no-vit items brings your health down, though there is some kind of threshold, and your health settles a bit higher than your "naked" health.

Preparation:

Killer should turn on Player health bars (default [D]), and memorize victim's 5% life number.

Victim swaps to and from no-vit gear. This should bring his life low, but likely not below 5%. See last paragraph for staying in range.

Killer makes sure the area is clean from shooters, elites, and other dangerous mobs.

Actual farming (we did this in Old Tristram Road with zombies, it is also doable with Carrion Nests and endless bats):

Victim singles out a monster and aggroes it.

After killer is sure the monster is the only one following the victim, he wastes the monster.

BOTH players get Savior callout.

Goto 1.

Every now and then one of the players will inadvertently step on a health globe. The victim has to stay below 5% life, so he should run out and take some hits. On the other hand, dropping too low is also dangerous - if you die and are revived, you'll be at full life and will have to swap again. Ideally, the victim should have a controllable regen gear or healing spells to stay at 4-5%.